1. Field of the Invention
The invention relates to a system for protecting electrical switchgear, switchboards, panelboards, motor controls and other electric equipment from arcing faults.
2. Description of the Prior Art
In electrical equipment arcing faults may occur due to loose connections, conductive dust accumulation on insulators, insulation failure due to overloading and unsuccessful interruption of short circuits causing conductive gases to emanate from circuit breakers or fuses. Such faults are very turbulent in that the energy released by the arc can generate a tremendous amount of heat and arc pressures in a matter of a few cycles. A copper or aluminum bus and its grounded parts can quickly be melted away if these faults are allowed to persist beyond the very narrow time frame representing the incipient stage thereof. The fault may occur as phase to phase or phase to ground, depending upon the cause.
The need for adequate fault detection and instantaneous warning apparatus is well recognized within the industry, as noted in a publication entitled Electrical Construction and Maintenance June 1973, pp. 53-72, the disclosure of which is hereby incorporated by reference. By way of illustration, the following passage is quoted from page 53:
"Under normal conditions, all current supplied by a transformer or network bus to a system returns to the source through the phase and neutral conductors. Current returning by any other path--through ground or through the system grounding conductor--indicates a lead from phase to ground somewhere on the system. Such a leak can be caused by insulation deterioration, moisture, dust or other contaminants, damage to the equipment, or human error. While the fault current may be too small to trip the system's overcurrent protective devices, arcing at the point of fault can generate high temperatures and result in serious damage to equipment within a very short time."
The problem of fault detection is especially acute on low volatage systems. If an arc circuit impedance is high the current fluctuation is very low and difficult to detect. Different types of devices are presently used in the art to detect low-voltage electrical ground to phase faults. Among the ground to phase protection devices are broken-delta ground fault protection, ground sensor protection, ground return protection, and residual ground fault protection. The broken-delta device relies upon the measurement of voltage across broken delta-connected current sensors. In this arrangement a voltage will appear across the broken delta only when a ground fault develops in the system. The ground-sensor protector is based on a combination of a donut-type current transformer which surrounds all three or four outgoing conductors and a specific over current relay. The current transformer produces an output proportional to the ground fault component of the total outgoing current. The ground return protection mechanism relies on a baror donut-type current transformer and a conventional relay. The current transformer measures the ground fault current as it returns to the source of power. Finally, the residual ground fault protector measures the outgoing ground fault current in the residual circuit of three phase current transformers. Each of these protection devices has its own particular advantages and limitations. But all of them share the same limitation of relying on electrical changes in the equipment to activate the means such as a relay for opening the circuit to stop current flow.
Because the prior art protection devices rely on current sensing, there must be sufficient current change to activate them. However, the current magnitude of arcing faults may be so small that the usual phase overcurrent protection does not respond at all or only after a considerable time delay. This is particularly true where the current load is large; in such cases the degree of current changes with a fault may be too small for existing equipment to detect as a fault, i.e., current sensing systems lack the capability to respond quickly enough. To prevent damage to the equipment, the monitoring techniques should respond to incipient fault failure, for example, within a few seconds of time.
Present breakers may also be ineffective when applied to switches or circuit breakers having motors or similar devices which are not functional below a given voltage. Since the possibility exists that a phase to ground or phase to phase fault could pyramid or cascade to levels which would reduce the system operating voltage below the given voltage, these devices have not been effective in some circumstances and it often occurs that the electrical equipment to which they are attached suffers severe, either heat, fire or smoke damage before the electrical faults are detected and the circuit is shut off.